In the field of polymer production, silylated polymers are generally known to be useful as components of coatings, adhesives, sealants and other elastomeric products. The production of silylated polymers has traditionally employed a process whereby a prepolymer having reactive functional groups is reacted with a silylating agent, having groups reactive with the prepolymer, to produce a silylated polymer.
Commercially useful silylated polymers include moisture-curable polymers such as for example moisture-curable silylated polyurethane polymers. A process for preparing the silylated polyurethanes involves reacting a polyurethane prepolymer, as a first intermediate product, having terminal reactive groups with a silylating agent, such as for example an isocyanato-functional alkylalkoxysilane, to produce a silylated polyurethane composition, as a second intermediate product, having silylated polyurethanes that are endcapped with alkoxysilyl groups. Huang, et al. discloses one such process in US Patent Application Publication No. US2007/0100108.
These processes, and the silylated polymers produced from them, have several characteristics which render them problematic and which in part drive the need to develop better processes for producing silylated polymers. For example, the viscosity of the produced silylated polymer composition tends to erode over time such that the viscosity observed immediately after production of the silylated polymer does not remain the same, but rather increases over time. This phenomenon is due in part to the inability of the conventional process to bring the silylating reaction to completion. That is, when the prepolymer with reactive functional groups are reacted with the silylating agent in a continuous process, for example, this reaction is too slow. This results in a silylated polymer composition where some of the reactive functional groups on the prepolymer and some of the reactive groups on the silylating agent remain unreacted in the composition. Over time, these unreacted reactive functional groups will continue to react in the composition, resulting in viscosity creep. As used herein, the term “viscosity creep” refers to changes in the viscosity of a composition, such as the silylated polymer composition, over time as the composition ages. This generally refers to an increase in viscosity as the composition ages.
Another drawback of these processes is that the components in the produced silylated polymer composition that have unreacted reactive functional groups will function similarly to plasticizers. In this regard, they produce deteriorations in the mechanical properties of the silylated polymer composition, that would otherwise not be present or be present to a lesser degree. For example, the components with unreacted reactive functional groups cause a comparative reduction in tensile strength, shore hardness and modulus.
The unreacted reactive functional groups also contribute to variability in the produced silylated polymer compositions. This variability adds unwanted processing costs and losses for the manufacturer and for the consumer. It also results in unfavorable customer opinions as customers are likely to feel less confident that the produced moisture-curable silylated polymer meets their desired specifications.
Still another drawback of these processes is that the long production times, catalysts and high temperatures used to produce the silylated polymers lead to color formation.
One approach to force the reaction to completion is to use a large excess of the silylating agent in the conventional process relative to the prepolymer having reactive functional groups. The drawback is that the process is inefficient, requiring the use of large excesses of expensive silylating agents that remain unreacted in the silylated polymer composition, and therefore do not contribute to the silylation reaction. The unreacted silylating agent affects the viscosity and mechanical properties of the moisture-curable silylated polymer composition.
There remains a need for processes that can produce silylated polymers with reduced process and product variability, does not require large excesses of silylating agent to complete the silylation reaction and has low color.